X-ray spectrometry systems are used for detecting, measuring and analyzing X-ray emissions from, for example, a scanning electron microscope (SEM). As is known, X-ray emission spectra measured by an X-ray spectrometry system are frequently used in analyzing the elemental composition of materials and are generated by measuring the energies of a great number of emitted X-rays and plotting the numbers of emitted X-rays against the measured energy in a histogram. Emission lines characteristic of chemical elements or isotopes appear as peaks in the resulting histogram, and the pattern and heights of these peaks can be used to determine the composition.
A typical X-ray spectrometry system includes the following four main components: (1) a detector, (2) a pre-amplifier, (3) a pulse processor, and (4) a computer-based analyzer. The detector, which usually takes the form of a semiconductor sensor of some type, converts an incoming X-ray into a very small current pulse, typically on the order of tens to thousands of electrons, with a duration of about tens to a few hundreds of nanoseconds. The pre-amplifier amplifies the current pulse output by the detector and typically converts it into a voltage signal in the range of tenths of millivolts up to a few hundreds of millivolts. The pulse processor receives the pre-amplifier signal and generates a numeric representation of the X-ray photon energy through an integration process. The computer-based analyzer accumulates the X-ray photon energies output by the pulse processor into a spectrum or plot of the number of X-rays detected against their energies.
The portion of the pulse processor that derives the X-ray energy from the pre-amp signal is commonly referred to as the main channel. As noted above, the main channel derives X-ray energy using a signal integration process, called pulse shaping, which can take any of several mathematical forms approximated either by analog circuitry or by digital signal processing. The pulse processor has a user-selectable time, variously called the time constant, the shaping time or the rise time (for convenience, the term “shaping time” will be used herein), which determines the amount of signal integration that is performed in the main channel. In addition, a pulse processor parameter called dead time, referred to as DM herein, is proportional to the shaping time, with the proportionality constant being determined by the particular pulse shaping function that is used. If a second X-ray arrives (i.e., a pulse corresponding thereto is output by the pre-amplifier) within a time period equal to the dead time DM after arrival of a first X-ray, then both the first and second X-ray are discarded by the pulse processor because the response of the main channel pulse shaper will be distorted and its maximum will no longer be an accurate measure of the energy of either X-ray. This phenomenon is known as pulse pile-up. Thus, the main channel dead time DM is the amount of time that it takes for the main channel of the pulse processor to accurately and unambiguously measure the energy of a single X-ray. If a second X-ray arrives before the dead time interval expires, pile-up has occurred and no measurement of X-ray energy is made.
In addition, pulse processors output a signal, referred to herein as the main channel dead time signal, which is in an active state whenever an X-ray pulse is being processed in the main channel. We will describe an active-high system, in which the active state is indicated by a high logic voltage, but those of ordinary skill in the art will recognize that active-low logic is equally feasible. FIG. 1 illustrates an output 5 of the pre-amplifier of a current X-ray spectrometry system employing digital triangle shaping when a single X-ray is received at time T1, with no subsequent pile-up (i.e., with no arrival of a second X-ray within the main channel dead time DM). As seen in FIG. 1, in such a case, the main channel dead time signal 10 will go high at time T1, and will remain high for a time period equal to the dead time DM, after which it goes low again. FIG. 1 also shows the output 15 of the pulse processor which represents the energy of the X-ray. FIG. 2 demonstrates what happens when pile-up occurs. In particular, FIG. 2 shows the output 5 of the pre-amplifier when a first X-ray is received at time T1, and a second X-ray is received at time T2 wherein (T2−T1)<DM (i.e., pile-up). As seen in FIG. 2, in such a case, the main channel dead time signal 10 will go high at time T1 (when the first X-ray arrives) and will remain high for a time period equal to the dead time DM following the arrival of the second X-ray at time T2, after which it goes low again at time T3 (this is true provided the pulse processor's dead time following any X-ray's arrival is constant; such pulse processors are referred to as “paralyzable” in the art, an example of which is the Saturn model pulse processor from X-ray Instrumentation Associates in Hayward, Calif.). In other words, the high state of the main channel dead time signal 10 during pile-up will be extended by DM after the arrival of each X-ray in the sequence. Because this is a pile-up situation, each of the X-rays will be discarded and the pulse processor will not output an energy value.
In order to detect pulse pile-up, pulse processors may also be provided with one or more additional processing paths (i.e., in addition to the main channel) known as fast channels or pulse pile-up channels. The fast channels have very short shaping times, and thus very short dead times DF, as compared to the main channel. Like the main channel, each fast channel has associated therewith a fast channel dead time signal which is in a high state whenever the fast channel is processing an X-ray. Because the fast channel dead times DF are much shorter than the main channel dead time DM, the fast channels are much more likely to produce distinct fast channel dead time signal pulses for each of a number of X-rays arriving close together in time.
Moreover, the shaping time (and thus dead time) of any pulse processing channel (main or fast) determines the lowest energy X-ray which can be detected in that channel, known as the detection threshold for the channel. In particular, detection threshold is inversely proportional to shaping time while the minimum detectable time separation, which defines pile-up recognition performance, is directly proportional to shaping time. FIG. 3 shows the progression of noise levels and detection thresholds from the main channel to the longest fast channel through the shortest fast channel. X-rays lower in energy than the main channel threshold Tmain are undetectable. If more than one fast channel is provided, each fast channel uniquely detects an energy band, shown in Portion 3a of FIG. 3 as Band 1, Band 2 and Band 3, which is the difference between its detection threshold and that of the fast channel with the next shorter shaping time. Portions 3b-3e of FIG. 3 show an idealized, noise-free preamplifier output step function resulting from the arrival of an X-ray, the digital filter shape typical for a digital triangle shaper, and the output from each of the digital filters for a hypothetical system with one main channel and three fast channels. Note that the noise fluctuation range of the outputs of each of the digital filters, shown as the bottom trace of portion 3a of FIG. 3, becomes larger as the width of the shaping filters becomes shorter. The main channel digital filter shape typically has a period of constant negative weight, followed by a small gap of zero weight, followed by an equal period of constant positive weight. The gap, which gives a short flat top to the otherwise triangular the digital filter output, is wide enough to cover the finite rise time of the preamplifier step, and is required to avoid including the rapidly-changing preamplifier signal samples in the rise time in the weighted output of the digital filter, which would degrade the accuracy of the energy measurement. The fast channels, although shown in portions 3c-3e with such a gap, do not necessarily need one because they are not used for energy measurement. Current state of the art X-ray spectrometry systems will typically be able to distinguish X-rays energies as low as 100-200 eV from noise in the main channel, but the detection threshold of the shortest fast channel is much higher, typically up to 1,000-2,000 eV. Some existing pulse processors have as many as three fast channels to improve overall pile-up rejection performance in the range between 100 and 1,000 eV.
Currently, the fast channels of pulse processors are only used internally by the pulse processor to detect pile-up. In older analog pulse processors, an inhibit signal is generated when pile-up is detected which prevents the external analog-to-digital converter from digitizing the peak of the main channel output pulse. In newer digital pulse processors, an internal logic signal is generated when pile-up is detected which prevents succeeding stages of digital processing from capturing the distorted pulse. This approach, however, ignores the fact that it is possible to derive additional useful information from the main and fast channels. Thus, there is a need for processing methods that derive useful information from the main and fast channel outputs.